Exhaust driven turbochargers can include a rotating shaft carrying a turbine wheel and a compressor wheel where the shaft is typically rotatably supported within a center housing (e.g., intermediate a compressor and a turbine) by one or more lubricated bearings (e.g., oil lubricated). During operation, exhaust from an internal combustion engine drives a turbocharger's turbine wheel, which, in turn, drives the compressor wheel to boost charge air to the internal combustion engine.
During operation, a turbocharger's rotating assembly may reach rotational speeds in excess of 100,000 rpm (e.g., some may reach rotational speeds of 250,000 rpm or more). To handle such high speeds, a turbocharger's center housing rotating assembly (CHRA) requires balance and adequate lubrication. Factors such as noise, vibration and harshness (NVH), as well as efficiency, are often interrelated and must be within acceptable limits. As an example of interrelatedness, vibration can generate noise and reduce efficiency. Further, under dynamic conditions, such as an increase in exhaust flow, axial thrust forces can cause contact between various CHRA components. Contact can cause wear, which, in turn, can alter balance, leading to increased noise, vibration, etc., and reduced efficiency.
Turbocharger bearing systems may offer both support and damping to control motion of a turbocharger shaft, for example, to help isolate vibrations from rotating parts while allowing the turbocharger shaft to spin, for example, at speeds that may be about 60 times faster than a maximum engine speed (e.g., consider a diesel engine). A turbocharger bearing system may help ensure turbocharger operational efficiency by keeping frictional losses and NVH low such that energy from the engine exhaust gas is available to drive the turbocharger. Where operational conditions may vary, a turbocharger bearing system may be selected to help balance low-power losses with an ability to control forces applied by varying mechanical loading (e.g., thrust and other forces).
As to turbocharger bearing system hydrodynamics, fluid (e.g., oil or other lubricant) may lubricate components and also influence motion of a turbocharger shaft. As an example, a “fully-floating” bearing system can include a journal bearing that supports a shaft using an outer film disposed between a bore wall of a center housing and an outer surface of the journal bearing and an inner film disposed between an inner surface of the journal bearing and an outer surface of the shaft. In such an example, the journal bearing may rotate (azimuthally) at approximately one-half the speed of the shaft and move axially and radially (i.e., the journal bearing is fully-floating).
As to a “semi-floating” approach, an anti-rotation mechanism may act limit rotation (azimuthally) of a journal bearing or, for example, an outer race of a REB assembly. As an example, a semi-floating journal bearing or a semi-floating REB assembly may support a shaft using, in part, an outer oil film disposed between an outer surface of the journal bearing or an outer surface of the REB assembly and a bore wall of a center housing where the outer oil film acts as a “squeeze film”, for example, to damp undesirably shaft motions.
As an example, a turbocharger may include one or more rolling element bearing (REB) assemblies, which may be, for example, one or more ball bearing assemblies. An REB assembly can include an outer race, an inner race and rolling elements disposed between the inner and outer races (e.g., in a raceway or raceways). For example, consider an REB assembly that includes a unitary outer race and a two-piece inner race fit to a turbocharger shaft (e.g., a shaft and wheel assembly (SWA) where rolling elements allow for rotation of the shaft and two-piece inner race with respect to the outer race). In such an example, the outer race of the REB assembly may be “located” in a bore of a housing such as a center housing (e.g., disposed between a compressor housing and a turbine housing). As an example, to axially locate an outer race in a bore of a center housing, a counter-bore and a plate may be positioned at a turbine side and a compressor side of the center housing where each forms an opening with a diameter less than an outer diameter of the outer race. In such an example, the REB assembly may be placed in the bore followed by receipt of a shaft (e.g., a SWA) or, for example, the REB assembly may be fit to the shaft (e.g., a SWA) and then inserted into the bore (e.g., as a unit that includes the REB assembly and the shaft). Further, an anti-rotation mechanism may be provided that locates the outer race in the bore of the center housing by limiting rotation of the outer race (e.g., azimuthal direction). In such an example, the REB assembly may be “semi-floating”, for example, having an ability to move in a radial direction where radial clearances between an outer surface of the outer race and a bore surface of the center housing provide for squeeze film formation (e.g., one or more lubricant films).
Various examples of technologies, techniques, etc., described herein pertain to assemblies, housings, bearing assemblies, etc.